VF is a potentially life threatening cardiac tachyarrhythmia producing an immediate loss of blood pressure. In a normally functioning heart, the muscle fibers are stimulated by a wave-like electrical excitation originating in the sino-atrial node in the right atrium. The excitation then proceeds to the atrium and then to the ventricles of the heart. This wave-like excitation then triggers the ventricular muscle fibers by producing a sequential depolarization of adjacent cells, thereby effecting an efficient contracting and pumping action which is the normal mechanical activity of the heart. In certain individuals and under certain conditions, the organized wave-like pattern of electrical excitation becomes interrupted and VF results, this being a disorganized, random contraction and relaxation of the fibers of the ventricle. During VF, the muscle fibers are electrically depolarizing and repolarizing in a random manner thereby resulting in a chaotic twitching of the ventricular muscle with no effective pumping of blood being accomplished. This inevitably results in loss of consciousness of the patient and a high probability of death if appropriate therapy is not given to the patient.
By the application of a sufficient discharge of electric current to the ventricular muscle fibers, it is possible to depolarize an adequate number of the fibers at the one time to re-establish synchrony within the heart thereby enabling the ventricles to resume their normal rhythmic pumping activity.
VF must be treated within a short period of time following onset or the patient may die. A widely used treatment of VF's today is the delivery of a sufficiently powerful external electric shock to the ventricles of the heart. At present, there is an identifiable population of patients who survive an episode of VF due to prompt therapy. Although these patients may survive their first episode of VF due to the efforts of responsive hospital attendants, their long term chances of survival are not high. It is well known that within minutes of the onset of VF, irreversible changes begin to occur in the brain and other vital organs. It is therefore desirable to effect defibrillation as quickly as possible. For these patients who are becoming increasingly more identifiable, an alternative treatment now becoming more and more widely used is the implantable automatic defibrillator device which combines defibrillation and both bradycardia and antitachycardia pacing in a single device.
Prior to defibrillators, there existed devices for applying pacing pulses to the heart known as pacemakers. These were initially developed to electrically stimulate hearts that were unable to beat at a rate sufficient to maintain a life sustaining cardiac output. The first devices delivered electrical stimuli at a fixed rate regardless of the heart's function or the body's physiological needs.
At a later time devices were developed that stimulated the heart only if it failed to beat above a predetermined rate. Such devices sensed the electrical activity of the heart, usually in the right ventricle. Later developments saw the introduction of pacemakers that sensed and stimulated in both the right atrium and ventricle.
Additionally, pacemakers were introduced in order to obtain a measure of the body's physiological needs and which responded by altering the paced rate to meet the demand, for example, by sensing the respiratory rate and then increasing the heart rate as the respiratory rate increased. Such a pacemaker is disclosed in U.S. Pat. No. 4,702,253 to Nappholz et al. In this device, electrodes are placed in a blood vessel in the vicinity of the patient's pleural cavity, a known current field is established in the blood, and the blood impedance in the field is measured. The impedance is a function of the pleural pressure which, in turn, is a function of the patient's minute volume.
A major step in the pacemaker field saw the development of devices that electrically sensed the presence of a ventricular tachyarrhythmia and delivered a defibrillating D.C. shock to revert the heart to a normal rhythm. More advanced devices were developed that attempted to pace hearts undergoing a supra-ventricular or ventricular tachyarrhythmia back into a normal rhythm. This technique is known as antitachycardia pacing (ATP).
A further step included the development of automatic cardiac defibrillator devices which sense and analyze the electrical activity of the heart. Such a device is described in U.S. Pat. No. 3,857,398. The electrical activity of the heart has typically been detected by a pair of electrodes placed in or around the heart. This method enables detection of an ECG showing a record of R and T waveforms indicating stages of electrical depolarization and repolarization of the ventricles of the heart.
Combined devices have been developed that can act both as pacemakers and as arrhythmia control systems. These devices are able to pace a heart that is beating too slowly, to cardiovert/defibrillate a heart and to pace a heart undergoing a ventricular tachyarrhythmia, back into a normal rhythm.
An example of such a combined implantable device is described in U.S. Pat. No. 4,940,054, of Richard Grevis and Norma L. Gilli entitled "Apparatus and Method for Controlling Multiple Sensitivities in Arrhythmia Control Systems Including Post-therapy Pacing Delay".
The above mentioned device is a microcomputer based arrhythmia control system. The device is able to be programmed to many different bradycardia pacing modes. It uses a telemetric link to communicate with the physician. Variables such as the bradycardia support pacing rate and the atrio-ventricular (AV) delay can be programmed to suit the needs of the recipient of the device. However, such parameters can only be altered by a telemetric link. There is no provision for the device to adjust its programmed parameters in a learning response mode.
The use of a telemetric link allows not only the reprogramming of a device, but also the interrogation of a device by a clinician. Some devices are also fitted with vibrating warning devices to indicate to the patient certain error states within the device and/or malfunctions of the heart. The idea is to hasten the patient's presentation to the clinician to allow interrogation of the device.
It was found that existing devices which relied solely on electrical sensing in order to determine the state of the cardiac function still had some inherent limitations. For example, these devices have at times been confused by the presence of electrical noise in their sensing circuits thereby resulting in difficulty in distinguishing a VF. Even when the ECG classification is correct, the heart may not be pumping sufficient blood. It was found to be more effective by adding to the sensing capabilities certain manipulations of the right ventricular pressure signal as described in the co-pending patent application of Kenneth A. Collins, Ser. No. 481,364, filed Feb. 16, 1990, now U.S. Pat. No. 5,083,563, entitled "An Implantable Automatic and Haemodynamically Responsive Cardioverting/Defibrillating Pacemaker" which relates to a device that adds the ability to transduce haemodynamic compromise to a cardioverting/defibrillating pacemaker. It includes the provision of switching to the best mode of pacing for a given cardiac state by sensing the right ventricular filtered peak-to-peak amplitude (RVFPPA) or the right ventricular pulse pressure function (RVPPF), as well as the electrical activity of the right ventricle.
There still remain many shortcomings in existing devices. Of particular note is their lack of speed and accuracy in the detection of complex arrhythmias as well as in the diagnosis and the application of appropriate therapy. The recipients of existing cardioverter/defibrillators and cardioverting/defibrillating pacemakers may still face the risk of the inappropriate delivery of defibrillation therapy. Such therapy is not without risk of damage to the myocardium. Furthermore, unwarranted discharge of the device causes pain to the conscious patient, instilling great anxiety, as well as shortening the life of the device's batteries. Besides the risk of false positives, there is also the risk of false negatives when a needed therapy fails to be delivered. This can be a life threatening situation to a patient.
A further problem with existing defibrillator devices is that their use is resisted by some cardiologists who prefer the use of toxic beta-blockers, such as amioderone, despite the fact that the arrhythmia may still not be under control, as a preference to an implantable defibrillator device which has a large delay between the detection of a tachyarrhythmia and the application of therapy to a patient. A large delay results in the patient losing consciousness, thereby having a profound effect on his ability to continue driving a motor vehicle. This situation could therefore be extremely hazardous to the driver's own life as well as the lives of others. Furthermore, the loss of a driver's licence is a very undesirable fate in itself. The reason for such a large delay in existing implantable defibrillator devices is due not only to the diagnostic component but also to the large delay in the charging time of the defibrillator capacitor.
Another problem with present implantable combined pacemaker-defibrillator devices is the high level of power consumption involved. Such devices, which are typically microprocessor controlled, use a continuous high level of power due to the continual "wakened" state of the detection, diagnostic and therapy control phases of the device. A high level of power consumption significantly reduces the life of the battery, which in turn places trauma on the patient due to the need to have an early battery replacement in the implantable device.